The present invention relates to a compact zoom lens system having a zoom ratio of about 2 and a short total length so that it can be suitably used for lens shutter cameras etc. and, more particularly, to a zoom lens system comprising three positive, positive and negative units.
In recent years, zoom lenses more reduced in size and weight have been desired to meet an increased demand for making lens shutter cameras have a zooming function. For lens systems used for such lens shutter cameras, it is not required to make sure of a specific back focus, unlike lens systems mounted on single-lens reflex cameras. However, too snort a back focus would give rise to an increase in the diameter of the image-side lens. On the other hand, too long a back focus would lead to an increase in the lens's overall length with the result that it would be impossible to achieve any zoom lens compact in size.
For instance, the lens system set forth in Japanese Provisional Patent Publication No. 62(1987)-78522 is a zoom lens for lens shutter cameras, which comprises a first unit having a positive refractive power, a second unit having a positive refractive power and a third unit having a negative refractive power. This conventional zoom lens is comparable to a two-unit zoom lens in that it has a telephoto ratio of about 1.3 at the wide angle end, but the lens diameter of the first unit is too large. This zoom lens also has a zoom ratio of some 1.5. Some examples of the zoom lens system, which has a zoom ratio of ca. 2 with equally distributed positive, positive and negative refractive powers and wherein the first unit is made integral with the third unit during zooming, are set forth in Japanese Provisional Patent Publications Nos. 1(1989)-93713 and 2(1990)-50117. These prior art examples have a telephoto ratio of as large as about 1.4 and about 1.5 at the wide angle ends, respectively, and are increased in the overall lengths with increases in the lens diameters of the first units as well.
The zoom lens set forth in the above Japanese Provisional Patent Publication No. 62-78522 is comparable to a two-unit zoom lens in that its telephoto ratio at the wide angle end is about 1.3, but although it is reduced in the overall length, yet the lens diameter of the first unit is so increased that it is disadvantageous for making a camera body on which it is mounted compact in size. The conventional zoom lenses set forth in Japanese Provisional Patent Publications Nos. 1-93713 and 2-50117, which have a zoom ratio of about 2 that is the same as that of the present invention and wherein the first components are movable together with the third components toward the object side, but they have a telephoto ratio as large as about 1.4 and about 1.5, respectively, and are increased in the overall lengths. In addition, they are not said to be compact in size, because the lens diameters of the first units are large.